A reflection type phase modulator designed to a characteristic impedance requires an ideal circulator of the same characteristic impedance. When a practical circulator deviates from an ideal one, the deviation leads to problems dependent upon the magnitudes. For example, referring to FIG. 1a, a carrier V.sub.1 is applied to a circulator 100 which has an isolation problem. M is a vector representation of a phase modulated carrier having magnitude of unity and phase proportional to an applied modulating signal voltage. I is an isolation vector having a magnitude which is typically smaller than 0.1 in a practical circulator. It can be seen therefore that EQU V.sub.2 = (I+M) .sup.. V.sub.1 EQU V.sub.2 /V.sub.1 = I + M = M - (-I) = M'
M' being the resulting vector and PA1 .vertline.M.vertline. = 1 and .vertline.I.vertline. &lt;&lt; 1
Referring to FIG. 1b, if the phase angle of the isolation vector I is not fixed, the resulting signal V.sub.2 has distortion as well as phase sensitivity variation.
Ferrite which is typically used in circulators as non-reciprocal magnetic material usually has quite high temperature drift in magnetic property. The saturation magnetization, 4.pi.M.sub.s which is approximately inversely proportional to characteristic impedance of the ferrite, can vary more than 10% over a temperature range of 0.degree. to 55.degree. C. As a result phase modulators exhibit inherent temperature characteristics when used with circulators.
The invention eliminates such effects by providing a 180.degree. phase shift within one section of a two section phase modulator between one port of a circulator and one phase modulator. Temperature or frequency dependent efffects occurring in one circulator and phase modulator are balanced out by similar effects occurring in the other circulator and phase modulator.